The present invention is directed to pulse generating apparatuses. Pulse generation is an important function for many electrical devices and products. Pulses are used for many purposes including, by way of example and not by way of limitation, driving pulse width modulation circuitry. Pulses provided must be of predictable and consistent duration and period to provide a reliable timing circuit or modulating circuit or other feature of a product using the pulses.
Some prior art pulse generating apparatuses used long chains of series-connected delay units, known as delay chains, to establish desired delay intervals for generating pulses. However, in today's products there is a trend toward requiring smaller more compact circuitry and minimizing die space or “real estate” required by a circuit or apparatus is a design objective of increasing importance. One response has been to use a feed forward circuit arrangement by which a delay chain is used twice. By such an arrangement a single delay chain is established on a die and is used twice so that greater delays may be realized in a circuit using less die space than was required by earlier circuits not employing a feed forward technique.
A problem with such prior art devices has been floating nodes (see FIG. 1). Reliance on capacitance alone at a node to maintain circuit values such as a predetermined voltage is not reliable because without actively maintaining the desired value at the node, the charge will bleed off. Today's circuitry operates at sufficiently fast speeds that there is insufficient capacity at circuit nodes to maintain the desired logic values (e.g., voltages values) without driving the node. The circuit nodes cannot reliably maintain the required circuit values and, as a consequence, the circuit performance is also unreliable. This unreliability may be manifested, for example, in spurious pulses or truncated pulses from unpredictable coupling with a node. Said another way, unpredictable coupling with a node or unpredictable charging or discharging of a node in a circuit may cause the circuit to generate pulses having unpredictable shape or duration or other unpredictable characteristics.
There is a need for an apparatus and method for generating pulses that is compact and involves no floating nodes.